Final answer:
In sickle cell anemia, valine is substituted for glutamic acid at position 6 of the beta-globulin chain, causing a change in protein structure and function.
Step-by-step explanation:
In sickle cell anemia, the amino acid glutamic acid is substituted by valine at position 6 of the beta-globulin chain. This substitution causes a change in protein structure and function, leading to the formation of abnormal hemoglobin molecules and the characteristic sickle-shaped red blood cells.
The substitution is caused by a mutation in the gene encoding the hemoglobin protein, resulting in the substitution of valine for glutamic acid.